Flexographic printing systems typically comprise a rotatable printing cylinder (sometimes referred to as a “printing roller”) and an anilox roller for transferring ink to the printing cylinder. The printing cylinder is configured and continuously rotated with respect to a moving web so as to repeatedly print an image on a moving web. In conventional flexographic printing systems, the rotational speed of the printing cylinder is synchronized with the web line-speed. Hence, the size of the image and image repeat length (i.e. the distance between common points of two adjacent repeat images) is determined by the useful printing circumference of the printing cylinder. The theoretical limit of the size of the image and image repeat length is the maximum viable circumference of the printing cylinder. Typically, the entire printing cylinder surface is used for printing. Alternatively, a section of the printing cylinder circumference may be blank and non printing. This non-printing region is provided to delineate between individual printed images and to facilitate the joining of different pattern segments.
Accordingly, it is not possible for conventional flexographic printing systems to print images with a size and repeat length that is larger than the circumference of the printing cylinder. For example, a flexographic printing system having a printing cylinder with a circumference of 1 m can not print images with a repeat length greater than 1 m. Hence, a conventional flexographic printing system can not print images with a “wall height” repeat (typically 2.4 m or more).
Typically, large repeats (images having a large size and repeat length) have been obtained using alternative printing method such as the so-called “flat printing method” by means of flat stencils or the so-called “block printing method” by means of printing blocks. Although any size of image or repeat length may be achieved, the mechanical process of manufacture is laborious and the rate of production thereof is limited.
The problem of restricted image size has also been previously solved by reducing the rotational speed of the printing cylinder with respect to the web line-speed so as to print a stretched or elongated image on the web. This type of printing process is commonly referred to as “slip” printing. Although the image is larger than the printing region of the printing cylinder, the image produced by slip printing is considered to be an inferior quality.
Designers are creating designs that are becoming ever more challenging to print. For example, designers are creating designs having a large size format, remotely spaced images, random images and/or multiple colours. In many instances it is not possible to reproduce these designs using a conventional flexographic printing system due to the image size limitations, repeat length restrictions and the number of print stations required. Hence, to date, these challenging print designs are often only produced using digital printing technologies as opposed to flexographic printing technology. However, digital printing technologies have their own limitations and can for example, only be used on certain substrates and by using a limited range of inks and ink technologies.
One particularly challenging design for printing is a large format design (e.g. for wallpaper) which has a large almost continuous design presented over the whole wall length with multiple repeated images at relatively large repeat separations. Using a conventional flexographic printing process to try and achieve this design would require large numbers of print stations to build up the design in stages. In practice this arrangement would be unsuitable because it would be inherently difficult to control for quality, it would expensive and relatively inflexible.
There is therefore a need for a new flexographic printing method and flexographic printing system to address or overcome one or more of the problems discussed above.